1. Field of the Invention
The present invention relates to a method for detecting the position of the rotor magnetic pole of a synchronous motor, which is used as a drive source in a variety of machines and devices, such as machine tools controlled by numerical control (NC) devices, industrial machines, robots, and so forth.
2. Description of the Related Art
A synchronous motor can generate torque by supplying an electric current to a stator excitation phase, which possesses a certain angle (a 90-degree electrical angle) relative to the position of a rotor magnetic pole. To excite a stator excitation phase such that it possesses a 90-degree angle relative to this rotor magnetic pole position requires an understanding of the physical relationship between the rotor magnetic pole position and a stator excitation phase. Ordinarily, the detection of this magnetic pole position is detected by a sensor, such as an encoder or resolver mounted to the rotor of an electric motor.
Because the absolute position of a rotor is needed to detect this magnetic pole position, a high-priced encoder, resolver or the like is required. Further, when mounting a sensor to an electric motor, it becomes necessary to align the sensor with the motor magnetic pole position, but the problem is that this alignment is difficult, and consequently causes motor production efficiency to deteriorate, and is costly.
As a method of solving this problem, in the past, there was used a detection method, which applies direct current electrical current to the 0 degree stator excitation phase, making the place where the rotor stops the rotor magnetic position. However, it is extremely dangerous to employ this method in a state in which a motor is mounted to a machine because at detection time the rotor is moving in any direction at an electrical angle of up to 180 degrees.
An object of the present invention is to provide a synchronous motor rotor magnetic pole position detection method, which enables a rotor magnetic pole position to be detected safely and easily.
To achieve the above-mentioned object, the present invention is a rotor magnetic pole position detection method for a synchronous motor, which comprises a magnetic pole-equipped rotor, an exciting winding-equipped stator, and a rotor position-detecting sensor, and repeats the following series of steps from (a) to (d).
(a) A step for supplying electric current to a prescribed excitation phase of the stator,
(b) A step for acquiring the direction of movement of a rotor by the above-mentioned application of electric current,
(c) A step for estimating the rotor magnetic pole position based on the acquired direction of movement,
(d) A step for specifying a prescribed excitation phase at which electric current is to be supplied the next time based on the estimated magnetic pole position of the above-mentioned rotor.
Furthermore, the present invention is constituted so as to comprise a step for writing to a non-volatile memory the difference between a rotor magnetic pole position ultimately obtained by repeating the above-mentioned series of steps from (a) to (d), and a rotor position detected by the sensor at that time.
In the above-mentioned Step (c), a rotor magnetic pole position is estimated based on rotor movement direction in accordance with a phase range that is divided into two parts at an excitation phase at which electric current is supplied. Further, in the above-mentioned Step (d), there is specified a prescribed excitation phase, which further divides a phase range in which it is estimated that a rotor magnetic pole position is located. By repeating the above-mentioned Steps (a)-(d), a rotor magnetic pole position is ultimately detected by steadily narrowing the phase range in which the rotor magnetic pole position is estimated to exist.
Further, to reduce the movement of the rotor, the present invention is constituted such that after the above-mentioned Step (b), an electric current of a reverse polarization to the electric current supplied in Step (a) is supplied to a prescribed excitation phase, returning the position of the rotor to the original position.
Furthermore, a rotor magnetic pole position can be determined more accurately by adding the amount of rotor movement during detection of the above-mentioned rotor magnetic pole position, as a correction quantity, to the above-mentioned estimated rotor magnetic pole position. Because generated torque declines as the stator excitation phase and phase of the rotor magnetic pole position draw nearer, the present invention is constituted such that the electric current value supplied to the above-mentioned prescribed magnetic pole position is changed at the time of repetition.
According to the method of the present invention, because a rotor magnetic pole position can be detected without rotating the rotor very much, it is possible to detect a rotor magnetic pole position even in a state in which a synchronous motor is mounted to a machine, thus enabling a rotor magnetic pole position to be detected safely and easily.